Graphite susceptor



United States Patent [72] Inventors Thomas J. Clark Troy;

Howard W. Brown, St. Clair Shores, Mich. [211 App]. No. 631,820 [22]Filed Apr. 18, 1967 [45] Patented Dec. 22, 1970 [73] Assignee GeneralElectric Company a corporation of New York 54 GRAPHITE suscEr'roRPrimary ExaminerDavid Klein Attorneys-Harold J. Holt, Frank L.Neuhauser, Melvin M.

Goldenberg and Oscar B. Waddell ABSTRACT: A susceptor for inductionheating comprising a body of porous graphite having a surface layer ofpyrolytic graphite which penetrates into the pores of the porous body tostrongly mechanically interlock the pyrolytic graphite surface layer tothe porous body. The pyrolytic graphite surface layer provides achemically inert impermeable surface which rapidly and uniformly spreadsthe heat generated in the susceptor, and the interlock assures againstseparation of the surface layer or cracking from mechanical or thermalshock. The susceptor is particularly useful for the epitaxial growth ofsemiconductors and integrated circuits. The susceptor is preferably madeby pyrolytically depositing the pyrolytic graphite on the porousgraphite body first at a relatively low temperature and at lowcarbonaceous gas pressure and then at increased temperature andcarbonaceous gas pressure to complete the deposit. This assures thedesired depth of penetration of the pyrolytic deposit to provide themechanical interlock and also a dense, smooth, continuous outerpyrolytic graphite surface and thereby provides the desired combinationof chemical and physical properties for use of the susceptor inepitaxial growth processes.

GRAPHITE SUSCEP'IOR This invention relates to a composite graphitesusceptor, particularly useful for the epitaxial growth ofsemiconductors and the like, and to a method for making same.

In epitaxial growth processes for the manufacture of semiconductors andthe like, it is necessary to heat a semiconductor chip, such as a chipof silicon metal, to high temperature to deposit thereon a thin layer ora plurality of thin layers of other materials. To accomplish this it isconventional to heat the chip by supporting it on an electricallyconductive body within an evacuated container of quartz or otherdielectric material surrounded by an induction heating coil, theconductive body functioning as a susceptor for the induction heating tothereby heat the chip. After the chip is brought to the desiredtemperature vapor deposition onto the chip can then proceed by admittingthe desired vapor to the container.

The susceptor used in such processing must not only be electricallyconductive but must also have extremely high heat resistance and must bechemically inert and of extremely high purity to assure against adversecontamination of the semiconductor chip being processed. At the presentstate of the art it is the common practice to use silicon carbide coatedgraphite as the susceptor. Such a susceptor has numerous disadvantages.First, there is a significant mismatch between the thermal coefficientsof expansion of silicon carbide and graphite in addition to whichsilicon carbide itself does not have good thermal shock resistance.Hence, such a susceptor cannot be thermal cycled rapidly and even whenthermal cycled slowly, cracking frequently occurs. Adding to the problemis the fact that silicon carbide is a rather brittle material and itoften occurs that upon cracking the silicon carbide debris is thrownonto the semiconductor chip being processed thereby resulting in scraplosses. Further, silicon carbide is expensive in the pure form and thisadds significantly to the expense of such susceptors.

It is the principal object of the present invention to provide asusceptor having improved properties of thermal and mechanical shockresistance, chemical inertness, impermeability, high purity and rapiduniform surface heat conduction. Another object is the provision of amethod for manufacturing such susceptors at reasonable cost.

Briefly, these objects are accomplished in accordance with the inventionby a susceptor which comprises a body of porous electrographite having asurface layer of pyrolytic graphite which penetrates into the pores ofthe electrographite body to provide a strong mechanical interlockbetween the pyrolytic graphite surface layer and the electrographitesubstrate body. Further in accordance with the invention, such asusceptor is manufactured by pyrolytically depositing the pyrolyticgraphite onto the electrographite body first at a temperature of fromabout l,l C. to 1,600 C. with carbonaceous gas pressure of less thanabout 1.2 mm. Hg pressure, and then continuing the pyrolytic graphitedeposition at a temperature above l,600 C. and at a carbonaceous gaspressure about 1.5 mm. Hg pressure. This assures good depth ofpenetration of the pyrolytic graphite into the pores of the porousgraphite body so that there is a strong mechanical interlock while atthe same time providing a smooth dense continuous surface layer ofpyrolytic graphite. Susceptors so manufactured demonstrate 7 excellentthermal and mechanical shock resistance by reason of the mechanicalinterlock between the pyrolytic graphite surface layer and the porousgraphite substrate; chemical inertness along with surface impermeabilityand chemical purity by reason of the characteristics of the pyrolyticgraphite; and high efficiency particularly by reason of the excellentsurface thermal conductivity of the pyrolytic graphite. The susceptorscan be thermal cycled rapidly and can be used repeatedly without damageor significant loss of any of the desired properties. In effect, asusceptor made in accordance with the invention demonstrates acombination of desirable chemical and physical properties comparable tothose attainable with a 100 percent pyrolytic graphite body but atgreatly reduced cost since the bulk of the susceptor consists ofinexpensive electrographite.

Other objects, features and advantages of the invention will appear moreclearly from the following detailed description thereof made withreference to the appended drawings in which:

FIG. 1 is a cross-sectional view of apparatus used for manufacture ofthe susceptors;

FIG. 2 is a cross-sectional view, in enlarged scale, of a portion of theapparatus shown in FIG. 1;

FIG. 3 is a greatly magnified sectional view of a surface portion of asusceptor made in accordance with the invention;

FIG. 4 is a sectional view of apparatus incorporating a susceptor ofthis invention and used for the manufacture of semiconductors by theepitaxial growth method.

Referring now to FIG. I, the apparatus shown comprises a generallycylindrical casing 10 having a closure plate 12 which is removablysecured as by bolts or a suitable hinge and latch. A viewing window 14enables inspection of the deposition operation within the casing. A bodyof insulating material 16, such as carbon black, defines an innercylindrical chamber the walls of which are formed by a graphite cylinder18 and top and bottom graphite plates 20 and 22 respectively. Aninduction heating coil 24 surrounds the insulating material 16, thegraphite cylinder 18 functioning as a susceptor whereby intense heat isgenerated within the cylinder 18 by reason of the passage of electriccurrent through the induction coil 24.

Extending through the heating chamberdefined by the cylinder 18 and itsend plates is an inner tube 26 of graphite having graphite pegs 28extending radially inwardly therefrom. The graphite bodies 30 desired tobe treated are supported by these pegs as shown in FIG. 2. That is, eachgraphite body is formed with a small cylindrical opening 32 which ispressed over the conical end of a support peg. If desired, the conicalend of the support peg can be formed with a plurality of small slots 34to assure admission of carbonaceous gas into the opening 32.

An opening in plate 22 accommodates an inlet conduit 36 forthe flow ofcarbonaceous gas into and through the inner graphite tube 18, the upperend of the tube 18 being open to the interior of the casing whereby thenondeposited products of the pyrolysis of the carbonaceous gas'can existthrough the outlet conduit 38. Hence, in operation the carbonaceous gas,such as methane or a mixture of methane and hydrogen, is admittedthrough tube 36 to the interior of the assembly consisting of graphitetube 18 and the graphite bodies 30 which assembly is intensely heated bythe heat generated by the cylinder 16. Pyrolysis of the carbonaceous gasthereby occurs with resultant deposition of the pyrolytic graphite onthe intensely heated graphite bodies 30, the hydrogen and other gaseouspyrolysis products being withdrawn from the chamber through the outletconduit 38.

For optimum properties in the finished articles, the electrographitebodies being processed should have a density not in excess of about 1.9g./cc. and preferably a density of about 1.7 to 1.9 g./cc. and a poresize distribution which peaks at from I to microns. With smaller poresit is difficult to attain the required pyrolytic graphite penetrationinto the surface to accomplish the desired strong interlock between thepyrolytic graphite and the substrate body. With a pore sizesignificantly above the aforesaid range an excessively long depositiontime and thick pyrolytic deposit is required in order to provide therequisite smooth pyrolytic graphite outer surface.

To manufacture the composite graphite susceptor bodies the chamber isfirst evacuated to a pressure not in excess of about 0.1 mm. Hg and isheated to from l,l00 to l,200 C. by induction heating of the graphitecylinder 18. Then carbonaceous gas, preferably methane or a mixture ofmethane and hydrogen, is flowed through the graphite tube 26 at apressure of from 0.5 to 1.2 mm. Hg while the temperature is maintainedat from l,l00to l,600 C; This is continued for about 14 to 20 hoursafter which the temperature is gradually raised by about 300 to 800 C.,to within the range of 1,600 to 2, 100 C., whereby the carbonaceous gaspressure increases by about from 0.5 to 1 mm. Hg, to within the range of0.8 to

2.5 mm. Hg. The continued deposition at temperature above that used forthe initial deposition is continued for a total of about 6 to 10 hoursto thereby complete the pyrolytic graphite deposit. Heating andadmission of carbonaceous gas are then discontinued, followed by aquiescent cool-down to room temperature over a period of about 12 hours.

During the initial pyrolytic graphite deposition at relatively lowtemperature and carbonaceous gas pressure, penetration of the pyrolyticdeposit well into the porous surface of the electrographite body isaccomplished, the remainder of the deposition at higher temperature andcarbonaceous gas pressure providing the dense smooth outer surface skinto the pyrolytic deposit. To accomplish the necessary depth ofpenetration of the pyrolytic graphite deposit into the pores of theelectrographite body, at least 50 percent of the total period ofdeposition should be at the initial relatively low temperature andcarbonaceous gas pressure.

The resulting pyrolytic graphite deposit and its interlockedrelationship to the substrate electrographite body is illustrated inFIG. 3. The pyrolytic graphite deposit 40 extends well into the pores ofthe porous graphite body but has a smooth impermeable surface layer asshown at 42. The thickness of the surface layer of pyrolytic graphiteabove the surface of the porous graphite body should be from about 1 to15 mils and the depth of penetration of the pyrolytic graphite into thepores of the porous graphite body should be from 1 to 10 times thethickness of the surface layer. The pyrolytic graphite deposit is, ofcourse, in the form of laminae extending generally parallel to thesurface on which deposited, though this inherent feature of pyrolyticgraphite is not shown in the drawings.

The following specific example will serve to further illustrate theprocess for making the composite graphite susceptor bodies.

The furnace, as shown in FIG. 1, was evacuated to 0.01 mm. Hg and thengradually heated by induction to a temperature of 1,200 C. In theparticular furnace here being used the hot zone formed by the graphitetube 26 was 13 inches long with a diameter of 7 inches. With thetemperature at l,200 C. a mixture of hydrogen and methane was flowedthrough the hot zone at a pressure of 0.8 mm. Hg for 16 hours, the inletflow rate being 6 standard cubic feet per hour hydrogen and 2 stan dardcubic feet per hour methane. Then the temperature was gradually raised,over a period of 6 hours, to 1,800 C., whereby the carbonaceous gaspressure increased to about 1.5 mm. Hg (the inlet flow rate of methaneand hydrogen being maintained the same as above) and the deposition wascontinued at the l,800 C. temperature for an additional 2 hours. Thefurnace, while under vacuum of about 0.01 mm. Hg, was then cooled toroom temperature over a period of 12 hours after which the furnace wasbrought up to atmospheric pressure by gradual admission of air and thefinished composite graphite susceptor bodies removed. The bodies had asmooth continuous surface layer of pyrolytic graphite, such layer havinga thickness, above the surface of theporous graphite substrate, of about4 mils and and a depth of penetration into the surface of the porousbody of about 10 mils.

FIG. 4 shows the composite graphite susceptor body 44, made by theprocess as described'above, incorporated in apparatus for the epitaxialgrowth of a semiconductor. The chamber 46, generally made of quartz, isconnected to a vacuum pump through the opening 48 and to a source ofvaporized material for vapor deposition on the semiconductor chipsthrough the opening 50. The composite graphite susceptor body 44 is offlat annular shape with a plurality of circumferentially arrangedrecesses 52 in its upper surface. The semiconductor chips 54 desired tobe treated are positioned in the recesses. The susceptor body issupported on a quartz shelf 56 which rests on the bottom wall of thechamber 46. An induction coil 58 surrounds the chamber for heating ofthe susceptor body 44, generally to about 1,400 C. The various vaporizedmaterials and the type of layers applied to the chips in the epitaxialgrowth method arqwell known in the art and form no part of the presentinvention. The important point is that the composite graphite susceptorbody, as described above, provides excellent thermal and shockresistance and hence long life, chemical inertness to assure againstcontamination of the semiconductors, and excellent efficiency as asusceptor. By reason of the anisotropic properties of the pyrolyticgraphite surface layer, there is rapid uniform distribution of the heatgenerated.

it will be understood that while the invention has been described indetail with reference to a preferred embodiment thereof, variousmodifications may be made all within the full and intended scope of theclaims which follow.

We claim:

11. A susceptor for use in induction heating apparatus to support andheat material in the manufacture of semiconductor elements in the formof a composite graphite body, said composite graphite body comprising abody of porous graphite with a surface layer of pyrolytic graphite, saidpyrolytic graphite layer having a thickness exterior of the surface ofsaid porous body of from about 1 15 mils, and said pyrolytic graphitelayer extending into the pores of said porous body to a depth of 1--10times the thickness of that portion of the layer exterior of said porousbody to mechanically interlock said porous layer to said porous body.

2. A composite graphite body as set forth in claim 1 wherein said porousgraphite body has an initial density of from about 1.7 to 1.9 and has apore size distribution which peaks at from about 1 to microns.

3. In combination with an induction heating coil, a susceptor for saidinduction heating coil comprising a composite graphite body having asubstrate of porous electrographite and a surface layer of pyrolyticgraphite which extends into the pores of the substrate to therebymechanically interlock the pyrolytic graphite surface layer with theporous substrate.

4. A method for manufacturing a composite graphite body comprising thesteps of depositing pyrolytic graphite on a porous electrographite bodyby pyrolysis of a carbonaceous gas initially at a temperature of fromabout 1,100 to 1.600 C. and at a carbonaceous gas pressure of from about0.5 to 1.2 mm. Hg and then subsequently at a temperature of from about1,600 to 2,100 C. and at a carbonaceous gas pressure of from about 0.8to 2.5 mm. Hg.

5. A method as set forth in claim 4 wherein the period for the initialdeposition of pyrolytic graphite at from 1,100 to 1,600 C. and at acarbonaceous gas pressure of from about 0.5 to 1.2 mm. Hg constitutes atleast 50 percent of the total period of deposition.

6. A method as set forth in claim 5 wherein the period for the initialpyrolytic graphite deposition is from about 14 to 20 hours and whereinthe period for the subsequent pyrolytic graphite deposition at atemperature above that used for the initial deposition is about 6 to 10hours.

